Eighteen patients with appreciable obstructive lung disease and 18 patients without measurable mechanical lung impairment were given intravenous diazepam to permit endoscopy. Doses of diazepam required for comparable level of anesthesia were used. There was an increase in the heart rate, and there were decreases in the blood pressure, stroke volume, left ventricular stroke work, and cardiac output in patients with normal pulmonary function. Total peripheral resistance was elevated after 15 minutes. In patients with obstructive lung disease, diazepam produced insignificant changes in heart rate, diastolic blood pressure, and total peripheral resistance. Left ventricular stroke work, stroke volume, and systolic blood pressure were markedly reduced. There was a depression in alveolar ventilation, associated with an increase ill the arterial carbon dioxide tension and a decrease in pH. The only significant differences in the effects of diazepam on cardiopulmonary function were in diastolic blood pressure and left ventricular stroke work, which were more affected in the control subjects than in the patients. O2 saturation was affected more in the patients than in the control subjects.